Many times I've had companies start on a truck inventory. It has rarely worked. Here are some things to think about:
1) Can you afford to take a truck of the road for a 1/2 day to count the parts on it?
2) Who is going to do the counting?
3) Will a parts room person cost more then you save with better inventory control?
4) When 2 trucks go out on the same job how do you track which truck the inventory came from?
5) What do you do about unused salvage parts?
6) How do you resolve conflicts that arise when plumber A says he lent a part to plumber B?
Truck inventory or any type of detail inventory sounds like a great idea but usually fails. It is just to time consuming and expensive.